Aerobic Oxidation/Annulation Cascades through Synergistic Catalysis of RuCl3 and N-Heterocyclic Carbenes

Article abstract of DOI:10.1002/chem.201803254

Cooperative catalysis combining a transition metal with an N-heterocyclic carbene is challenging due to strong binding of NHCs towards late transition metals. We report the first example of synergistic catalysis by a chiral NHC and a coordinatively unsaturated ruthenium compound. RuCl₃ was found to mediate efficient aerobic oxidation of homoenolates generated from enals and the N-heterocyclic carbene. The resulting α,β-unsaturated acylazolium intermediate reacts selectively with 1,3-dicarbonyl compounds or ketones at either the β- or γ-carbon, yielding polysubstituted chiral lactones in high yield and with excellent enantioselectivity (up to 98 % yield, 94 % ee). This protocol can be applied to structurally sophisticated substrates.

Full text of DOI:10.1002/chem.201803254

A Journal of
Accepted Article
Title: Aerobic Oxidation/Annulation Cascades via Synergistic Catalysis
of RuCl3 and N-Heterocyclic Carbenes
Authors: Qian Wang, Jiean Chen, and Yong Huang
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To be cited as: Chem. Eur. J. 10.1002/chem.201803254
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10.1002/chem.201803254
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Aerobic Oxidation/Annulation Cascades via Synergistic Catalysis
of RuCl3 and N-Heterocyclic Carbenes
Qian Wang,[a] Jiean Chen*[a] and Yong Huang*[a]
Dedication ((optional))
Abstract: Cooperative catalysis combining a transition metal with an
N-heterocyclic carbene is challenging due to strong binding of NHCs
towards late transition metals. We report the first example of
synergistic catalysis by a chiral NHC and a coordinatively unsaturated
ruthenium compound. RuCl3 was found to mediate efficient aerobic
oxidation of homoenolates generated from enals and the N-
heterocyclic carbene. The resulting α,β-unsaturated acylazolium
intermediate reacts selectively with 1,3-dicarbonyl compounds or
ketones at either the β- or γ-carbon, yielding polysubstituted chiral
lactones in high yield and with excellent enantioselectivity (up to 98%
yield, 94% ee). This protocol can be applied to structurally
sophisticated substrates.
knowledge, synergistic catalysis involving an NHC and a
ruthenium with labile ligands is unknown.
Synergistic catalysis involving
a transition metal and an
organocatalyst has received intensive attention in recent years.[1]
Such combinations of two or more orthogonal catalytic
mechanisms has led to the discovery of a number of novel
chemical transformations. Several privileged classes of
organocatalysts, secondary amines[2] and chiral phosphoric
acids[3] for example, have been shown to work seamlessly with
various transition metals, but viable combinations of transition
metals (TM) and N-heterocyclic carbines (NHC), an important
class of organocatalysts, have seen limited success. Owing to
their electron-rich σ-donor property, NHCs are very strong binders
to the majority of TMs, especially late transition metals, and more
so than phosphines.[4] NHCs are often used to moderate the
catalytic activity of metal catalysts. Consequently, the
organocatalytic function of NHCs is quenched by being their
tightly bound to TMs. So far only a handful of TMs have been
shown to be compatible with NHCs derived from common
triazolium salts, and serving as an independent organocatalyst in
the same reaction system.[5,6,7] Synergistic systems are limited to
a narrow reaction scope. In the case of ruthenium, NHCs are only
applicable to fully saturated octahedral complexes, due to the
strong coordination of NHCs to Ru. Rovis used [Ru(bpy)3]Cl2 to
tether a photocatalytic circle with formation of an NHC-catalyzed
Breslow intermediate (Scheme 1a).[8] Studer reported single-
electron oxidation of the Breslow intermediate by [Ru(bpz)3](PF6)2
(Scheme 1b),[9] but low conversions were obtained for the
corresponding homoenolate intermediates. To the best of our
Scheme 1. Synergistic catalysis by NHC and Ru.
The α,β-unsaturated acylazolium species is a versatile
reaction intermediate that allows highly enantioselective
annulation reactions using various bisnucleophilic reagents.[10]
Unsaturated acylazolium intermediates are commonly generated
by oxidation of the corresponding homoenolate, formed by
treating an enal with NHC. Often, stoichiometric oxidants such as
quinones[11] or MnO2,[12] are employed in such reactions. From a
green chemistry perspective, the use of molecular oxygen or air
represents one of the cleanest oxidation strategies. Our group has
long been interested in aerobic oxidation reactions using various
transition metals such as Pd, Cu, Ir and Au.[13] We attempted to
devise a Ru/NHC synergistic protocol for highly efficient oxidation,
using oxygen as the terminal oxidant, and converting
homoenolate to an α,β-unsaturated acylazolium and applying this
to the synthesis of chiral heterocycles (Scheme 3c).
A major side-reaction to this aerobic process is β-
protonation of the enal substrates, as a result of slow oxidation by
O2. Reacting the enal (1a) with a catalytic amount of an NHC
precursor and a base in the presence of MeOH and O2 leads to
formation of the saturated methyl ester (3a’) as the major product
in 32% yield (Table 1). The desired oxidation product, methyl
cinnamate (3a), is isolated in 9% yield, a result suggesting that
a
the β-protonation pathway is faster than direct aerobic oxidation
[a]
Ms. Q. Wang, Prof. Dr. J. Chen and Prof. Dr. Y. Huang
State Key Laboratory of Chemical Oncogenomics, Key Laboratory
of Chemical Genomics, Peking University, Shenzhen Graduate
School
Shenzhen, 518055 China
E-mail: chenja@pkusz.edu, huangyong@pkusz.edu.cn
14]
of the homoenolate.[6b,
In order to accelerate the oxidation
pathway, we tested several saturated Ru species that are known
to be single electron transfer (SET) oxidants. Interestingly, co-
catalysis by Ru(bpy)3Cl2 leads exclusively to the β-protonation
product (3a’). Replacing bipyridine (bpy) with ligands that are
more electron-withdrawing failed to favor chemoselectivity over
oxidation. The use of a stoichiometric quantity of Ru reagent
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yields similar results, suggesting those ligand-saturated Ru(II)
complexes are ineffective oxidants for the homoenolate.[15] The
reduction potential (E1/2) of Ru(bpy)32+ vs SCE is -1.33 V in MeCN.
The oxidation potential of the homoenolate is not known, but an
analogous aza-homoenolate intermediate was reported by Rovis
to be oxidized at -0.49 V vs SCE in MeCN. These Ru(II)
complexes in the photo-excited state have been extensively
explored as strong oxidants. However, low yield for both oxidation
and β-protonation is obtained when the reaction was carried out
under blue LED radiation. Decomposition of 1a is serious under
light. RuCl3 is a highly effective catalyst for aerobic oxidation of
tertiary amines[16] and excellent turnover by molecular oxygen has
been reported. Indeed, the combination of NHC and RuCl3 leads
to high conversion of the enal (1a) to 3a, demonstrating opposite
chemoselectivity compared to Ru(bpy)32+. The α,β-unsaturated
ester (3a) was isolated in 80% yield, and should be noted that
although RuCl3 is a common precursor for ligand exchange, this
reaction represents the first example of synergistic catalysis of
NHC by a Ru complex with labile ligands. The selected examples
in Table 1 demonstrate that the catalytic synergy could efficiently
generate a series of unsaturated esters compounds (products 3b-
3g).
We next examined the feasibility of linking the aerobic
oxidation step to an annulation reaction using 1,3-diketones.
Existing methods for reactions involving β-ketoesters usually
require exotic terminal oxidants,[11a, 11c] and the newly established
RuCl3/NHC system could emerge as an alternative, aerobic
approach. The (3+3) annulation reaction between the enal (4a)
and 2,4-pentanedione (5a) was examined under RuCl3/NHC
synergistic catalysis (Table 2). The competing aldol reaction
between 4a and 5a and related decomposition was significant.
Both
base
and
solvent
are
essential
for
the
homoenolate/oxidation/annulation cascade (Table 2, entries 2-8).
Excellent yields and ee were obtained using weak sodium acetate
in 1,4-dioxane. Bode’s NHC catalyst (2b) afforded the highest
enantioselectivity and control experiments showed both RuCl3
and oxygen are required for this aerobic transformation.
Table 2. Aerobic (3+3) annulation by RuCl3/NHC catalysis.[a]
Table 1. Aerobic oxidation of enals catalyzed by NHC and Ru.[a]
entry
NHC
2b
2b
2b
2b
2b
2b
2b
2b
2c
base
solvent
yield (%)[b]
trace
5
ee (%)[c]
ND
27
1
DABCO
DBU
PhMe
entry
1
Ru
atmosphere
3a (yield, %)
3a’ (yield, %)
2
PhMe
—
O2
O2
O2
O2
Ar
9
32
73
48
43
80
35
8
3
K2CO3
K2CO3
K2CO3
K2CO3
K3PO4
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
PhMe
19
85
2+
2
Ru(bpy)3
4
4
DCE
50
75
2+
3
Ru(bpm)3
16
20
0
5
THF
23
77
2+
4
Ru(bpz)3
Ru(bpy)3
Ru(bpy)3
RuCl3
6
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
38
87
5[b]
6[c]
7
2+
7
43
86
2+
O2
O2
18
80
8
86
92
9
44
-64
-35
91
10
11[d]
12[e]
13[f]
2d
2b
2b
2b
38
59
trace
98[h]
ND
93
[a] Reactions conditions: 0.1 mmol 4a, 0.2 mmol 5a, 0.01 mmol NHC
precursor, 0.01 mmol RuCl3 and 0.12 mmol base in solvent were stirred at
r.t. for 24 h. [b] Determined by GC analysis using n-decane as the internal
standard. [c] Determined by chiral HPLC. [d] Without 4Å MS. [e] Without
RuCl3. [f] 0.2 mmol 4a and 0.1 mmol 5a were used. [h] Isolated yield.
[a] Reactions conditions: 0.1 mmol 1a, 0.6 mmol MeOH, 0.01 mmol NHC
precursor 2a, 0.01 mmol ruthenium catalyst, 0.12 mmol NaOAc and 4Å MS
in 1,4-dioxane were stirred under a balloon at r.t. for 24 h. Yield was
determined by GC analysis using n-decane as internal standard. [b]
Stoichiometric Ru(bpy)32+ was used. [c] The reaction was carried out under
450 nm LED radiation. [d] Isolated yield.
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With the optimized reactions conditions in hand, we
surveyed the scope of substrates. Various cinnamaldehydes were
tested for the aerobic (3+3) annulation. The β-aryl group tolerates
a number of substituents with either electron-donating or -
withdrawing properties, regardless of the substitution pattern
(Table 3, products 6ba-6ia). Comparable yields and ee were
obtained for β-napthyl and phenanthryl substituted enals
(products 6ja-6ka). It should be noted that β-heteroaryl
compounds, including those containing basic nitrogen atoms, do
not interfere with RuCl3. The pyridyl analogue afforded the desired
trisubstituted lactone (6la) in 92% yield and 91% ee. Groups
sensitive to strong oxidants, such as furyl, thienyl and
benzothienyl, are not affected (products 6ma-6oa). A good yield
and ee is observed for a β-n-propyl enal (product 6pa),
demonstrating that the choice of enals is not limited to
cinnamaldehydes. Exclusive (3+3) annulation occurs for dienals
(product 6qa). Reactions involving substrates bearing
complicated, biologically relevant structural motifs further
demonstrates the robustness of this protocol (products 6ra-6ta).
Functional groups, including ketones, esters, ketals or ethers, are
well tolerated, suggesting this synergistic approach might be
appliable to late-stage modification of drug scaffolds.
RuCl3/NHC catalyzed aerobic annulation reaction. However, β-
keto amides fail to participate in such annulation process. Due to
preferred formation of the ketone enol, the cyclization occurs on
the ketone carbonyl (products 6ae-6mn). Both cyclopropyl and
allyl substituents are tolerated in this aerobic oxidation process.
In addition to methyl ketoesters, a number of higher order alkyl
analogues react smoothly. With inceased steric hindrance of the
ketone moiety, the reaction becomes slower. For example, when
an isopropyl ketone substrate was used, the desired lactone
product (6mn) was obtained in 80% yield and 84% ee. β-
ketoesters containing exotic structural fragments are well
tolerated (products 6mn and 6ao) and various functional groups
such as olefin, ether, acetal, Michael acceptor, basic
heteroarenes and even free OH are unaffected by the aerobic
conditions. High yields and ee were obtained in most cases.
Table 4. Scope of 1,3-dicarbonyls.
Table 3. Scope of enals.
The key oxidation step is believed to be a Ru-mediated SET
process. In order to determine the nature of the radical,
cyclopropyl substituted enals were tested. The relative
stereochemistries of the cyclopropanes are perfectly preserved
during both the oxidation and the annulation processes. The lack
of ring opening and isomerization products suggests the radical is
strictly localized at the carbonyl carbon, with little resonance to
the corresponding β-radical isomer. This result is consistent with
Chi’s findings using nitrobenzenesulfonic carbamate as a terminal
oxidant.[17a]
The scope of 1,3-dicarbonyl compounds was examined
next. Symmetrical 1,3-diketones react with similar conversion and
enantioselectivity (Table 4, products 6ab and 6nb). For
unsymmetric analogues, the cyclization occurs on the less
sterically hindered carbonyl (products 6ac and 6ad). In addition to
1,3-diketones, β-ketoesters are also competent substrates for the
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Scheme 4. Derivatization of the [3+3] annulation product.
The in-situ generated α,β-unsaturated acylazolium species
can be further elaborated, using β,β-disubstituted enals, for γ-
functionalization via the azolium dienolate intermediate. Upon
introducing a trifluomethyl ketone (8) to the Ru/NHC reaction, an
enantioselective (4+2) annulation occurs smoothly to yield δ-
chiral lactones with good conversion and selectivity (Table 5).[19]
Instead of acting as an electrophile, the α,β-unsaturated
acylazolium undergoes γ-deprotonation and adds to the electron-
deficient carbonyl at the γ-carbon. Mg(OTf)2 was identified as a
Lewis acid cocatalyst for further activation of the ketone. To the
best of our knowledge, this reaction is the first aerobic protocol for
this important class of molecules.
Scheme 2. Radical clock experiments.
A preliminary reaction mechanism is proposed in Scheme
3. The homoenolate is generated from the enal by reaction with
the NHC. RuCl3 oxidizes the homoenolate by an SET, in an
manner analogous to that proposed by Rovis, Chi, Studer and
17]
Sun.[9
The resulting radical cation is deprotonated to yield a
,
zwitterionic radical species. A second SET by Ru leads to the α,β-
unsaturated acylazolium, which undergoes (3+3) annulation with
the 1,3-dicarbonyl compound. Meanwhile, Ru(III) is recycled via
direct oxidation by molecular oxygen. Although aerobic oxidation
mediated by RuCl3 has been extensively studied in amine
oxidation, the mechanistic details remain elusive, especially for
the change of oxidation state of Ru during the catalytic cycle[16]
and alternative pathways involving high-valence Ru species, such
as oxoruthenium, cannot be ruled out.[16d] Further investigation of
this question is currently underway.
Table 5. Aerobic (4+2) annulation by Ru/NHC catalysis.[a]
[a] Reactions conditions: 0.2 mmol enal 1, 0.1 mmol ketone 8, 0.01 mmol
NHC precursor, 0.01 mmol RuCl3, 0.01 mmol Mg(OTf)2 and 0.12 mmol
NaOAc were stirred in THF for 24 h at r.t..
Scheme 3. Proposed aerobic oxidation of homoenolate.
In summary, we report the first synergistic catalysis
combining NHC and a coordinatively unsaturated Ru(III) species.
This catalyst system enables efficient aerobic oxidation of a
homoenolate intermediate, which can be tethered to either a
nucleophile or an electrophile to deliver (3+3) and (4+2)
annulation products, respectively. In this way, polysubstituted
lactones can be synthesized in a single step in high yield and
enantioselectivity. This transformation features a double SET
mechanism and uses molecular oxygen as the terminal oxidant.
Synthetic applications demonstrate this strategy as an efficient
tool for the derivatization of complicated structures.
The chiral lactone (6aa) can be converted to diverse chiral
scaffolds (Scheme 4). MCPBA oxidation affords
a
bicyclo[4.1.0]epoxide product (7a) in 74% yield and with excellent
diastereoselectivity.[18] Direct aminolysis, using benzylamine,
proceeds smoothly at 0 ºC to give the β-chiral amide (7b). The
analogous transesterification can be promoted with MeOH and
sodium acetate. The resulting ester (7c) is an excellent synthon
for chiral heteroarenes, products 7d and 7e.
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[10] a) S. De Sarkar, A. Biswas, R. C. Samanta, A. Studer, Chem. Eur. J.
2013, 19, 4664-4678; b) S. J. Ryan, L. Candish, D. W. Lupton, Chem.
Soc. Rev. 2013, 42, 4906-4917; c) J. Mahatthananchai, J. W. Bode, Acc.
Chem. Res. 2014, 47, 696-707; d) D. M. Flanigan, F. Romanov-
Michailidis, N. A. White, T. Rovis, Chem. Rev. 2015, 115, 9307-9387; e)
M. H. Wang, K. A. Scheidt, Angew. Chem. Int. Ed. 2016, 55, 14912-
14922; fC. Zhang, J. F. Hooper, D. W. Lupton, ACS Catal. 2017, 7, 2583-
2596.
Acknowledgements
This work is financially supported by the National Natural Science
Foundation of China (21602007 and 21572004), the China
Postdoctoral Science Foundation (2017T100010) and Shenzhen
Basic Research Program (JCYJ20170818085510474 and
JCYJ20170818085438996).
[11] a) S. De Sarkar, S. Grimme, A. Studer, J. Am. Chem. Soc. 2010, 132,
1190-1191; b) S. De Sarkar, A. Studer, Angew. Chem. Int. Ed. 2010, 49,
9266-9269; c) Z. Q. Rong, M. Q. Jia, S. L. You, Org. Lett. 2011, 13, 4080-
4083; d) A. G. Kravina, J. Mahatthananchai, J. W. Bode, Angew. Chem.
Int. Ed. 2012, 51, 9433-9436; e) J. Mahatthananchai, J. Kaeobamrung,
J. W. Bode, ACS Catal. 2012, 2, 494-503; f) C. Zhao, D. Guo, K.
Munkerup, K. W. Huang, F. Li, J. Wang, Nat. Commun. 2018, 9, 611.
[12] a) B. E. Maki, A. Chan, E. M. Phillips, K. A. Scheidt, Org. Lett. 2007, 9,
371-374; b) S. Lu, S. B. Poh, W. Y. Siau, Y. Zhao, Angew. Chem. Int. Ed.
2013, 52, 1731-1734; c) S. Lu, S. B. Poh, Y. Zhao, Angew. Chem. Int.
Ed. 2014, 53, 11041-11045.
Keywords: Synergistic Catalysis • Cooperative Catalysis •
Aerobic Oxidation • Ruthenium • N-Heterocyclic Carbene
[1]
a) Z. Shao, H. Zhang, Chem. Soc. Rev. 2009, 38, 2745-2755; b) C.
Zhong, X. Shi, Eur. J. Org. Chem. 2010, 2999-3025; c) A. E. Allen, D. W.
Macmillan, Chem. Sci. 2012, 3, 633-658; d) Z. Du, Z. Shao, Chem. Soc.
Rev. 2013, 42, 1337-1378.
[2]
a) Z. Wang, X. Li, Y. Huang, Angew. Chem. Int. Ed. 2013, 52, 14219-
14223; b) A. Quintard, T. Constantieux, J. Rodriguez, Angew. Chem. Int.
Ed. 2013, 52, 12883-12887; c) S. Krautwald, M. A. Schafroth, D. Sarlah,
E. M. Carreira, J. Am. Chem. Soc. 2014, 136, 3020-3023; d) L. Naesborg,
K. S. Halskov, F. Tur, S. M. Monsted, K. A. Jørgensen, Angew. Chem.
Int. Ed. 2015, 54, 10193-10197; e) R. R. Liu, B. L. Li, J. Lu, C. Shen, J.
R. Gao, Y. X. Jia, J. Am. Chem. Soc. 2016, 138, 5198-5201.
a) G. L. Hamilton, E. J. Kang, M. Mba, F. D. Toste, Science 2007, 317,
496-499; b) S. Mukherjee, B. List, J. Am. Chem. Soc. 2007, 129, 11336-
11337; c) G. Jiang, B. List, Angew. Chem. Int. Ed. 2011, 50, 9471-9474;
d) Z. L. Tao, W. Q. Zhang, D. F. Chen, A. Adele, L.-Z. Gong, J. Am. Chem.
Soc. 2013, 135, 9255-9258.
[13] a) W. Gao, Z. He, Y. Qian, J. Zhao, Y. Huang, Chem. Sci. 2012, 3, 883-
886; b) H. Chen, Z. Wang, Y. Zhang, Y. Huang, J. Org. Chem. 2013, 78,
3503-3509; c) W. Yin, C. Wang, Y. Huang, Org. Lett. 2013, 15, 1850-
1853; d) Z. Wang, L. Li, Y. Huang, J. Am. Chem. Soc. 2014, 136, 12233-
12236; e) Y. Zhang, Q. Wang, H. Yu, Y. Huang, Org. Biomol. Chem.
2014, 12, 8844-8850; f) Z. Wang, Z. He, L. Zhang, Y. Huang, J. Am.
Chem. Soc. 2018, 140, 735-740.
[3]
[14] a) B. E. Maki, A. Chan, K. A. Scheidt, Synthesis 2008, 1306-1315; b) B.
E. Maki, E. V. Patterson, C. J. Cramer, K. A. Scheidt, Org. Lett. 2009, 11,
3942-3945; c) M. H. Wang, D. Barsoum, C. B. Schwamb, D. T. Cohen,
B. C. Goess, M. Riedrich, A. Chan, B. E. Maki, R. K. Mishra, K. A. Scheidt,
J. Org. Chem. 2017, 82, 4689-4702; d) L. Zhang, P. Yuan, J. Chen, Y.
Huang, Chem. Commun. 2018, 54, 1473-1476.
[4]
[5]
a) S. Diez-Gonzalez, N. Marion, S. P. Nolan, Chem. Rev. 2009, 109,
3612-3676; b) G. C. Fortman, S. P. Nolan, Chem. Soc. Rev. 2011, 40,
5151-5169; c) D. Janssen-Muller, C. Schlepphorst, F. Glorius, Chem.
Soc. Rev. 2017, 46, 4845-4854.
[15] D. P. Rillema, G. Allen, T. J. Meyer, D. Conrad, Inorg. Chem. 1983, 22,
1617-1622.
a) R. Lebeuf, K. Hirano, F. Glorius, Org. Lett. 2008, 10, 4243-4246; b) M.
M. Ahire, S. B. Mhaske, Angew. Chem. Int. Ed. 2014, 53, 7038-7042; c)
Y. Bai, W. L. Leng, Y. Li, X. W. Liu, Chem. Commun. 2014, 50, 13391-
13393; d) Y. Bai, S. Xiang, M. L. Leow, X. W. Liu, Chem. Commun. 2014,
50, 6168-6170; e) K. Liu, M. T. Hovey, K. A. Scheidt, Chem. Sci. 2014,
5, 4026-4031; f) C. Guo, M. Fleige, D. Janssen-Muller, C. G. Daniliuc, F.
Glorius, J. Am. Chem. Soc. 2016, 138, 7840-7843; g) C. Guo, D.
Janssen-Muller, M. Fleige, A. Lerchen, C. G. Daniliuc, F. Glorius, J. Am.
Chem. Soc. 2017, 139, 4443-4451; h) S. Singha, T. Patra, C. G. Daniliuc,
F. Glorius, J. Am. Chem. Soc. 2018, 140, 3551-3554; i) S. Yasuda, T.
Ishii, S. Takemoto, H. Haruki, H. Ohmiya, Angew. Chem. Int. Ed. 2018,
57, 2938-2942.
[16] a) S. Murahashi, N. Komiya, H. Terai, T. Nakae, J. Am. Chem. Soc. 2003,
125, 15312-15313; b) M. North, Angew. Chem. Int. Ed. 2004, 43, 4126-
4128; c) S. Murahashi, N. Komiya, H. Terai, Angew. Chem. Int. Ed. 2005,
44, 6931-6933; d) S. Murahashi, T. Nakae, H. Terai, N. Komiya, J. Am.
Chem. Soc. 2008, 130, 11005-11012; e) C. Allais, O. Baslé, J. Grassot,
M. Fontaine, S. Anguille, J. Rodriguez, T. Constantieux, Adv. Synth.
Catal. 2012, 354, 2084-2088.
[17] a) N. A. White, T. Rovis, J. Am. Chem. Soc. 2014, 136, 14674−14677; b)
Y. Zhang, Y. Du, Z. Huang, J. Xu, X. Wu, Y. Wang, M. Wang, S. Yang,
R. D. Webster, Y. R. Chi, J. Am. Chem. Soc. 2015, 137, 2416-2419; c)
W. Yang, W. Hu, X. Dong, X. Li, J. Sun, Angew. Chem. Int. Ed. 2016, 55,
15783-15786.
[6]
[7]
a) K. Namitharan, T. Zhu, J. Cheng, P. Zheng, X. Li, S. Yang, B. A. Song,
Y. R. Chi, Nat. Commun. 2014, 5, 3982; b) J. Chen, P. Yuan, L. Wang,
Y. Huang, J. Am. Chem. Soc. 2017, 139, 7045-7051.
[18] J. Mo, L. Shen, Y. R. Chi, Angew. Chem. Int. Ed. 2013, 52, 8588-8591.
[19] a) J. Mo, X. Chen, Y. R. Chi, J. Am. Chem. Soc. 2012, 134, 8810-8813;
b) X. Chen, S. Yang, B. A. Song, Y. R. Chi, Angew. Chem. Int. Ed. 2013,
52, 11134-11137; c) X. Y. Chen, Q. Liu, P. Chauhan, D. Enders, Angew.
Chem. Int. Ed. 2018, 57, 3862-3873.
a) A. Axelsson, E. Hammarvid, L. Ta, H. Sunden, Chem. Commun. 2016,
52, 11571-11574; b) L. Ta, A. Axelsson, H. Sundén, Green. Chem. 2016,
18, 686-690.
[8]
[9]
D. A. DiRocco, T. Rovis, J. Am. Chem. Soc. 2012, 134, 8094-8097.
J. Zhao, C. Mück-Lichtenfeld, A. Studer, Adv. Synth. Catal. 2013, 355,
1098-1106.
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10.1002/chem.201803254
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Qian Wang,[a] Jiean Chen*[a] and Yong
Huang*[a]
Aerobic Oxidation/Annulation
Cascades via Synergistic Catalysis of
RuCl3 and N-Heterocyclic Carbenes
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